120 Merc thermostat issue?

Polypedon · Apr 25, 2012

Hello all,

I have a 1985 Chapparal 178 xl, and since purchasing last fall the temp gauge does not register at all. She runs good, have had her out a bunch of times going both slow and fast (tops out at about 30 mph at 3500 rpm) and the motor was just serviced a few weeks ago due to shift cable being out of adjustment - I did not ask them to look at the temp gauge!

My question: is it worth fooling around with the existing thermostat and gauge, or just replacing the thermostat since swapping it out does not look to be a big deal?

Thanks!

Bondo · Apr 25, 2012

Re: 120 Merc thermostat issue?

My question: is it worth fooling around with the existing thermostat and gauge, or just replacing the thermostat since swapping it out does not look to be a big deal?

Ayuh,.... Welcome Aboard,.... I'd Test the gauge by grounding out the sender wire at the T-stat housing, it should go to Full Hot...

achris · Apr 25, 2012

Re: 120 Merc thermostat issue?

Polypedon said:
Hello all,

I have a 1985 Chapparal 178 xl, and since purchasing last fall the temp gauge does not register at all. She runs good, have had her out a bunch of times going both slow and fast (tops out at about 30 mph at [size=+1]3500 rpm[/size]) and the motor was just serviced a few weeks ago due to shift cable being out of adjustment - I did not ask them to look at the temp gauge!

My question: is it worth fooling around with the existing thermostat and gauge, or just replacing the thermostat since swapping it out does not look to be a big deal?

Thanks!

That's a problem! Either there's something wrong with the engine or the prop's too big (pitch)... You need to get those revs up, or you'll be re-building the engine. That engine should be making 4200-4600 rpm at Wide Open Throttle....

Chris.......

Polypedon · Apr 25, 2012

Re: 120 Merc thermostat issue?

thanks for the tips, I will try grounding out the sender to check the gauge and go from there. The fuel gauge is also broken, I have been searching the forum and found similar helpful info to troubleshoot that issue, have yet to fix it tho!

Also, let me clarify about the rpms...she tops out at at least 4000 rpms, but I don't keep it there too long just to go easy on her. Once it planes out, it runs comfortably at 3,500 rpms at about 30 mph.

achris · Apr 25, 2012

Re: 120 Merc thermostat issue?

Polypedon said:
thanks for the tips, I will try grounding out the sender to check the gauge and go from there. The fuel gauge is also broken, I have been searching the forum and found similar helpful info to troubleshoot that issue, have yet to fix it tho!

Also, let me clarify about the rpms...she tops out at at least 4000 rpms, but I don't keep it there too long just to go easy on her. Once it planes out, it runs comfortably at 3,500 rpms at about 30 mph.

That's fine, but it needs to make AT LEAST 4200 at wide open, regardless of what you like cruising at. My boat cruises beautifully at 3000, but at full noise makes 4950 (44.3 knots)... Just the way I like it

...

Polypedon · Apr 25, 2012

Re: 120 Merc thermostat issue?

I grounded the tan sending wire at the thermo housing and the gauge pegged full, guess i am installing a new thermostat. Thanks for the tip.

markheck1 · Apr 25, 2012

Re: 120 Merc thermostat issue?

Are you talking about the Thermostat, or the Temp sender. It could be either one.....
Is there even a thermostat in it? Some people think since they are in a fresh water cooled boat they leave the thermostat out thinking it will cool better.

It could also be that the temp sensor is bad. You might try this.....go run the boat, get it to where it should be warm enough, shut it off....wait about a minute and turn the key on, and just wait......even if the thermostat is stuck open or not there it should build enough heat in the engine to move the gauge....if the gauge never moves at all I would lean toward the sender, if the gauge moves then I would lean towards the Thermostat

Also if you have an ohm meter, you can measure resistance at the temp sender to the block, with the engine off for safety of course. measure it cold....and measure it after you get it warm.....the resistance should be lower when warm.

NHGuy · Apr 26, 2012

Re: 120 Merc thermostat issue?

I believe you should have the thermostat gasket with a ground contact in it. Make sure it has good contact with metal on both sides. You can also check continuity from the sender body to ground with a meter. If you don't have a meter just hold a piece of metal or a wire from bare metal to the sender body, if the gauge moves to the actual temp just clean up your grounds.

Polypedon · Apr 26, 2012

Re: 120 Merc thermostat issue?

Good questions, I did not open the housing so I do not know if there is a thermostat in there. The ground out to test the gauge was to the sender, and having been out a bunch of times and never seeing the gauge move at all leads me to believe that it may in fact be the sender. Since I would have to take the entire housing apart to get to both the sender and the thermostat, at this point is would seem prudent to replace both (unless the thermostat looks brand new). I do not have an ohm meter. I've found a replacement thermostat and gasket for about $20, any suggestions for the sender? By the the way, is the temp sender and temp sensor part of the same assembly?

markheck1 · Apr 26, 2012

Re: 120 Merc thermostat issue?

Sender and Sensor basically the same thing...while I am not positive, it should just screw out of the housing.
Guessing between 20 and 40 to replace it.

On the thermostat...if you open it, replace it......I never re-use a thermostat in any vehicle.....usually because of anti-Freeze loss in vehicles if you have to replace it, but I would treat the boat the same way....

Edit: A quick search, guessing at your engine shows 806409T or an optional 49734 as the Sender. I wonder if Merc put their part numbers on the senders.....

Polypedon · Apr 26, 2012

Re: 120 Merc thermostat issue?

Agreed and thank you for the help. I will update when I replace everything.

Polypedon · Apr 27, 2012

Re: 120 Merc thermostat issue?

Pulled both the thermo and temp sender last light without much difficulty. By the looks of them, they are the original parts (see pic of temp sender). In replacing the gaskets and threaded parts, is there any sealant or "goo" that I should be using?

riptorn · Apr 27, 2012

Re: 120 Merc thermostat issue?

Of course or you'll never get it back out. Any hardware store (for heat). Make sure you are using the right bolts for the housing with the star washers (can't remember the right name) or use lock washers. The housing has to be grounded before the temp sensor will work.

pevaguy · Apr 28, 2012

Re: 120 Merc thermostat issue?

One suggestion is that when installing the sensor avoid using teflon tape as it can insulate it from being grounded making it not work.

Polypedon · May 7, 2012

Re: 120 Merc thermostat issue?

Well the new thermostat and temp sender went in without a problem. I took her out this weekend and hit about 4500 rpms at WOT for a few minutes, but mostly cruised around at nearly no wake speed. However the temp gauge never got higher than 140? (the gauge is graduated from 120 to 160 in this range and it sat right in the middle). I thought it would have been hotter after running around a little bit bur no. And I assume that is is working because after letting it sit for a while and then putting power back to the gauge (turning key on) it dropped to the low end of 120?. Does this all sound normal?

achris · May 7, 2012

Re: 120 Merc thermostat issue?

Polypedon said:
Well the new thermostat and temp sender went in without a problem. I took her out this weekend and hit about 4500 rpms at WOT for a few minutes, but mostly cruised around at nearly no wake speed. However the temp gauge never got higher than 140? (the gauge is graduated from 120 to 160 in this range and it sat right in the middle). I thought it would have been hotter after running around a little bit bur no. And I assume that is is working because after letting it sit for a while and then putting power back to the gauge (turning key on) it dropped to the low end of 120?. Does this all sound normal?

Yep...

Polypedon · May 8, 2012

Re: 120 Merc thermostat issue?

Thanks!

120 Merc thermostat issue?

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